A rotary member such as a drive shaft, gear and so on used to transmit a torque from a prime mover to a desired portion or a member is vibrated inevitably by a fluctuation of the torque or load, or by a friction. A frequency of the vibration is varied according to rotational speed, and high order vibrations such as secondary vibrations are also generated. Therefore, amplitude of vibrations is widened by resonance. As a result, noise is generated and durability of the rotary member is degraded. Examples of a device or mechanism for suppressing such vibrations of equipment for transmitting power by rotation is disclosed in Japanese Patent Laid-Open No. 7-280037 and Japanese Patent Laid-Open No. 2000-97843.
A vibration absorber taught by Japanese Patent Laid-Open No. 7-280037 comprises a chamber formed on an outer circumferential portion of a rotary member such as a flywheel, and a rolling member such as a roller (or a skid) held in the chamber. According to the teachings of Japanese Patent Laid-Open No. 7-280037, when the rotary member is rotated, the rolling member rolls in the chamber while entering into resonance with torsional vibrations thereby absorbing the torsional vibrations dynamically. Meanwhile, a dynamic damper taught by Japanese Patent Laid-Open No. 2000-297843 comprises a rolling mass on which a diametrically larger elastomer is mounted, and a storing chamber holding the rolling mass therein which is formed on an outer circumferential portion of a hub. According to the teachings of Japanese Patent Laid-Open No. 2000-297843, in case a centrifugal force acting on the rolling mass is weak, that is, in case a rotational speed of the hub is low, the rolling mass is kept away from an inner face of the storing chamber by the elastomer. To the contrary, in case the rotational speed of the hub is increased and the centrifugal force is thereby increased, the elastomer is compressed by the rolling mass, and the rolling mass rolls along the inner face of the storing chamber while being contacted therewith.
Specifically, according to the vibration absorber taught by Japanese Patent Laid-Open No. 7-280037, a diameter of the rolling member held in the annular chamber is smaller than a radius of the annular chamber. In the annular chamber, a V-shaped guide member capable of opening widely is arranged at the center portion in a rotatable manner, and the rolling member is held between the guide member and the inner face of the annular chamber. Therefore, in case great torsional vibrations are generated, the rolling member is rotated together with the guide member thereby absorbing the vibrations. To the contrary, in case the rotational speed is low, the rolling member is held between the inner face of the chamber and the guide member. Therefore, generation of noise and micro vibrations can be prevented. However, according to the teachings of Japanese Patent Laid-Open No. 7-280037, a volume of the rolling member is rather small in comparison with a capacity of the chamber. In addition, it is difficult to situate the rolling member certainly at the outer circumferential side in the chamber. For these reasons, vibration damping capacity of the vibration absorber taught by Japanese Patent Laid-Open No. 7-280037 is insufficient. Moreover, according to the teachings of Japanese Patent Laid-Open No. 7-280037, it is necessary to arrange the guide member in the chamber in a rotatable manner. Therefore, a number of constructional elements has to be increased by additional members for arranging the guide member. Thus, the vibration absorber taught by Japanese Patent Laid-Open No. 7-280037 has to be improved to avoid those disadvantages.
As described, according to the dynamic damper taught by Japanese Patent Laid-Open No. 2000-297843, the elastomer is interposed between the rolling mass and the inner face of the storing chamber. Therefore, the torque is transmitted through the elastomer, and the vibration damping capacity is thereby degraded. Moreover, it is rather difficult to set elasticity of the elastomer to a value possible to hold the rolling member away from the inner face of the storing chamber in a low-speed region, and also possible to damp the vibrations of target frequency. Further, the elastomer will be deteriorated with age, and if the elastomer is deteriorated, noise would be generated and the vibration damping capacity would be changed.